The present invention relates to a two-cycle engine, in particular for use as a driving engine in a portable, manually operated tool.
Due to their low power/weight ratio, two-cycle engines are particularly suited for use as drive motors in manually operated, portable tools such as chainsaws, cut-off machines, blowers, brush cutters, and the like.
Due to the manner in which the two-cycle engine operates, the inflowing fresh mixture displaces the waste gases, forcing them out of the combustion chamber into the outlet, whereby it is understood that a portion of the fresh mixture also flows out through the outlet without having undergone combustion. These so-called scavenging losses are responsible for waste gases with high concentrations of pollutants, in particular high amounts of hydrocarbons.
The object of the invention is to further develop a two-cycle engine of the above general type such that the scavenging losses are reduced, and thus the quality of the waste gases is improved, with a low degree of complexity in terms of construction.
This object is achieved inventively in that for the duration of the scavenging cycle largely fuel-free gas flows in the area of said combustion chamber near said outlet, and in that for the duration of the scavenging cycle fuel-rich gas overflows out of said crankcase in the area of said combustion chamber further away from said outlet.
The gas, which is largely free of fuel, usefully flows through channels near the outlet in the region near the outlet, while the fuel-rich gas that is needed for operating the internal combustion engine is usefully supplied via channels that are further away from the outlet. This means that the fuel-free gas can shield the outlet in the manner of a curtain of air so that the fuel-rich gas cannot flow out via the outlet. For this it is essential that largely exclusively fuel-free gas flow in for the entire period of the scavenging cycle, advantageously for the entire period that the channel near the outlet is open, and the fuel/air mixture for operating the combustion engine flows out of the crankcase exclusively via the channels further away from the outlet. The gas portions flowing out via the outlet are overwhelmingly the largely fuel-free gas, which is why it is possible to achieve good waste gas qualities with low proportions of hydrocarbons. Since the fuel/air mixture, and its oil, flow exclusively via the crankcase, good lubrication is ensured even when the quantity of oil is low. Supplying the fuel/air mixture exclusively via the crankcase makes it possible to reduce the quantity of oil added to the fuel, whereby the burned waste gases carry less pollutants.
The air-supplying channels that are close to the outlet are large in terms of volume, in particular several times larger in volume than the channels that are further from the outlet and that supply the fuel-rich gas. The volume of the air-supplying channels is provided structurally in a size in which it can receive the entire volume of air flowing into the combustion chamber during one gas exchange process. In this manner the fuel/air mixture entering the air-supplying channel from the crankcase during the gas exchange process is merely used for a propellant in order to force the air that is pre-positioned in the air-supplying channel near the outlet into the combustion chamber.
The valve via which the air flows out of an air intake member into the channel near the outlet is preferably a diaphragm valve, but can also be a check valve controlled by crankcase pressure.
In one preferred embodiment, a piston-controlled auxiliary window is arranged in the cylinder wall as the valve such that it is constantly covered by the piston skirt. Provided in the piston skirt itself is a connecting channel that in a pre-determined lift position of the piston interconnects the auxiliary window and the inlet window of an overflow channel near the outlet. This ensures that air supplied via the air intake member to the auxiliary window flows in via the connecting channel in the piston skirt into the overflow channel near the outlet, that is, the overflow channel is completely filled with air from the combustion chamber going in the direction of the crankcase. When the overflow channel near the outlet is opened, the pure air flows into the region near the outlet and forms a curtain that shields the outlet, preventing the inflowing fuel-rich gas further away from the outlet from flowing out.
In order to ensure that enough air comes in, the channels near the outlet advantageously communicate with an accumulator that is usefully filled with air via a diaphragm pump driven by oscillating crankcase pressure. During the entire gas exchange process, air flows out of the accumulator into the region of the combustion chamber near the outlet, whereby the fuel/air mixture is reliably shielded from the outlet.
In order to achieve control of the channels, in particular of the inlet channel supplying the fuel-rich gas or the overflow channels, arranged in the desired gas channel is a control valve actuated by crankcase pressure. The control valve is in particular a pressure-actuated valve slide that is force-actuated in its closed position by a spring.